Steam cracking of hydrocarbons is widely used to produce olefins such as ethylene, propylene, butenes (1-butene, cis- and trans-2-butene, isobutene), butadiene, and aromatics such as benzene, toluene, and xylene. In an olefin plant, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam. This mixture, after preheating, is subjected to severe thermal cracking at elevated temperatures (1500° F. to 1600° F.) in a pyrolysis furnace. The cracked effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule). This effluent contains hydrocarbons that are aliphatic, aromatic, saturated, and unsaturated, and may contain significant amounts of molecular hydrogen. The effluent is then further separated into various individual product streams such as hydrogen, ethylene, propylene, mixed hydrocarbons having four or five carbon atoms per molecule (crude C4's and C5's), and pyrolysis gasoline.
Crude C4's can contain varying amounts of n-butane, isobutane, 1-butene, 2-butene (cis- and/or trans-), isobutene, acetylenes (ethyl acetylene and vinyl acetylene), and butadiene. As used herein, “2-butene” includes cis-2-butene, trans-2-butene, or a mixture of both. See N. Calamur, et al., “Butylenes,” in Kirk-Othmer Encyclopedia of Chemical Technology, online edition, 2007.
Crude C4's are typically subjected to butadiene extraction or butadiene selective hydrogenation to remove most, if not essentially all, of the butadiene and acetylenes present. Thereafter the C4 raffinate (called raffinate-1) is subjected to a chemical reaction (e.g., etherification, hydration, dimerization) wherein the isobutene is converted to other compounds (e.g., methyl tertiary butyl ether, tertiary butyl alcohol, diisobutene) (see, e.g., U.S. Pat. Nos. 6,586,649 and 4,242,530). The remaining C4 stream containing mainly n-butane, isobutane, 1-butene, and 2-butene is called raffinate-2.
When the market demand for methyl tertiary butyl ether declines, the conversion of isobutene from raffinate-1 to methyl tertiary butyl ether may not be economical. It is desirable to produce other useful products from raffinate-1.
Methods for producing isoamylene and/or isoprene from C4 streams are known. U.S. Pat. No. 3,621,073 teaches a method of converting butenes to isoamylene through a series of steps including purification, isomerization of 1-butene to 2-butene at a temperature below 0° C., and a metathesis reaction of 2-butene and isobutene to produce isoamylene. The disadvantage of the process is that the isomerization reaction, even at below 0° C. only partially converts 1-butene to 2-butene, thus complicating the down-stream purification and reaction steps.
U.S. Pat. No. 4,085,158 teaches a method of producing isoamylenes from butenes comprising steps of separating 1-butene from a mixed butenes stream to produce a mixture of 2-butene and isobutene by molecular sieves adsorption and desorption, metathesis of 2-butene and isobutene, and isomerization of 1-butene to 2-butene. This method suffers from the high operating cost of the adsorption step due to the limited capacity of the adsorbent.